fallback to forwarded headers if the scheme is lying to us

fixes up heroku cedar issues where redirect url doesn't infer https

lib/warden-github/strategy.rb

@@ -48,16 +48,16 @@ def oauth_proxy
   end
 
   def callback_url
-    absolute_url(request, env['warden'].config[:github_callback_url])
+    absolute_url(request, env['warden'].config[:github_callback_url], env['HTTP_X_FORWARDED_PROTO'])
   end
 
-  def absolute_url(request, suffix = nil)
+  def absolute_url(request, suffix = nil, proto = "http")
     port_part = case request.scheme
                 when "http"
                   request.port == 80 ? "" : ":#{request.port}"
                 when "https"
                   request.port == 443 ? "" : ":#{request.port}"
                 end
-    "#{request.scheme}://#{request.host}#{port_part}#{suffix}"
+    "#{proto}://#{request.host}#{port_part}#{suffix}"
   end
 end